An electrochemical device, such as an electric double layer capacitor, made thinner by using a film package has been drawing attentions (see Patent Literatures 1 and 2). For the thinner electrochemical device, there is an increasing demand for: (1) reflow soldering of the electrochemical device to a circuit board; or (2) encapsulating the electrochemical device into an IC card.
This type of film package is generally fabricated of a laminate film comprising a heat resistant layer, a barrier layer, and a heat seal layer stacked in this order. Specifically, such a film package is fabricated by placing an electric storage element having terminals on a heat seal layer of the laminate film such that the tip portions of the terminals are protruded from an edge of the film; stacking another laminate film on the electric storage element such that a heat-seal layer thereof faces the electric storage element and then heating peripheries of the laminate films to integrate the heat-seal layers with each other by thermal fusion bonding. In another method of fabricating the film package, one laminate film is folded in two, instead of stacking two laminate films together.
In a typical process for reflow soldering an electrochemical device containing a film package to a circuit board, the circuit board having installed thereon the electrochemical device is placed into a reflow furnace. Therefore, temperature rise occurs to the electrochemical device in accordance with the temperature profile for the reflow soldering process, causing the temperature of the electrochemical device to rise to the peak or near-peak temperature of the reflow soldering.
In addition, in a typical process for encapsulating an electrochemical device containing a film package into an IC card, the electrochemical device is housed into a device housing unit formed on a core sheet and then a cover sheet is heat sealed to the top and bottom surfaces of the core sheet. In this process, temperature rise occurs to the electrochemical device in accordance with the temperature profile for the heat sealing, causing the temperature of the electrochemical device to rise to the peak or near-peak temperature of the heat sealing.
The heat seal layer of the laminate film used in the film package comprises a thermoplastic plastic such as polypropylene to facilitate heat sealing and to prevent temperature rise in the film package during heat sealing.
The melting point of this thermoplastic plastic is not high; therefore, in the process where the electrochemical device containing a film package is reflow soldered onto a circuit board or encapsulated into an IC card, the temperature rise causes softening or melting of “heat seal layers integrated with each other by thermal fusion bonding.” Further, in this process, the internal pressure of the film package is increased by the rise in steam pressure of an electrolyte, causing the electrolyte or a gas to leak from the softened or molten portion. The leakage of the electrolyte or gas may stain the periphery or degrade the function of the electrochemical device.
In an electrochemical device containing a film package, the tip portion of the terminal is guided outside through “heat seal layers integrated with each other by thermal fusion bonding.” The leakage of the electrolyte or gas tends to occur at the interface between the terminal and “heat seal layers integrated with each other by thermal fusion bonding.”